The present invention provides a process for working up or treating the reaction water and wash water are obtained during the preparation of dinitrotoluene (DNT) by the nitration of toluene. It is necessary to treat the reaction water and wash water to avoid losses of DNT, and to be able to send the process waste water for a biological work-up.
In the conventional processes for the preparation of dinitrotoluene (DNT) from toluene and a mixture of sulphuric acid and nitric acid (nitrating acid), the acidic reaction water distilled off in the sulfuric acid concentration step, and alkaline and acidic wash water from the purification of the DNT, are obtained as aqueous waste waters. In addition to mononitrotoluene and dinitrotoluene, this process waste water contains other nitration by-products such as, for example, mononitrocresols, dinitrocresols and trinitrocresols (hereafter referred to globally as nitrocresols), picric acid and nitrobenzoic acids. There are two main reasons why these substances have to be removed from the aqueous phases. First, with concentrations of DNT up to or exceeding 2.5 wt. % and of MNT as high as 1.5 wt. % being present in the process water, the disposal of the untreated waste water represents a loss of yield of desired product. Second, aromatic nitro compounds do not easily degrade in biological waste water treatment plants and have properties toxic to bacteria.
Different processes have been described in previous studies on the nitration of aromatic compounds with treatment of the aqueous phases formed.
U.S. Pat. No. 6,506,948 describes the nitration of toluene, wherein each of the aqueous phases obtained are extracted directly with toluene. U.S. Pat. No. 6,506,948 does not disclose or suggest settling of the organic materials prior to extraction. In the extraction process of U.S. Pat. No. 6,506,948, the aqueous acidic phases from the sulfuric acid concentration step, as well as an aqueous acidic phase and an aqueous alkaline phase from the DNT washing step, are extracted with toluene, with the toluene stream being introduced into the individual aqueous phases in succession. Then, the toluene stream is processed in the nitration process. Since the various waste waters are treated separately with toluene, each waste water stream requires its own mixer-settler.
U.S. Pat. No. 4,642,396 describes the transfer of nitration products from the aqueous acidic phase to the organic phase in the nitration mixture by the introduction of nitrogen monoxide. Any nitric acid still present is thereby reduced to nitrogen dioxide, which can be converted to nitric acid in a suitable downstream process. However, this process requires the handling of nitrogen dioxide, a substance foreign to the process, which obviously has to be utilized in another plant for the preparation of nitric acid.
The recovery of nitroaromatics (e.g. nitrobenzene) from alkaline wash water by extraction with the educt of the nitration process (benzene) is described in, for example, U.S. Pat. No. 4,241,229. This reference specifically relates to the nitration of benzene to nitrobenzene by the nitrating acid process. It also discloses the separation of nitroaromatics from waste water and wash water by steam stripping. Although it describes the recovery of product, it does not describe or suggest a further treatment of the salts of phenols, picric acid and organic acids which are dissolved in the alkaline wash water.
The treatment of nitrocresols which are separated from the product stream in the alkaline DNT washing step by oxidative degradation with nitric acid at elevated temperatures requires its own separate process stage. This is described in EP-A1-0 962 446.
In the context of the preparation of nitrobenzene, U.S. Pat. No. 4,230,567 likewise describes the degradation of nitrophenol in an additional process step at elevated pressure and temperature.
U.S. Pat. No. 4,597,875 also proposes a process for the separation of nitrocresol components from the dinitrotoluene product and the resulting wash water. Nitrocresols are removed from the DNT phase as water-soluble salts in an extraction step using alkaline wash water. After the separation of DNT, sulfuric or nitric acid is added to the alkaline wash water in order to precipitate the nitrocresol components. Then, after mechanical settling of the nitrocresol phase, the latter can be disposed of in a suitable combustion process. The nitric acid and sulfuric acid feed materials are hereby consumed for separation of the secondary components. A separate disposal of the nitrocresol components is additionally required.
U.S. Pat. No. 4,257,986 provides a process for the treatment of sulfuric acid obtained in the nitration of aromatic compounds with nitrating acid. To remove nitric acid, nitrous acid and organic impurities from this sulfuric acid, the latter is treated with the aromatic compound to be nitrated (educt) and with various reducing or oxidizing agents. This process also requires the use of additional substances which are foreign to the nitration process.
The object of the present invention is to provide a simple and economic process for the recovery of the nitration products contained in the various aqueous waste waters and for the separation and treatment of unwanted secondary components of the nitration. Surprisingly, this is achieved by means of steps that are simple in terms of process technology, compared with existing processes, and without using additional process steps and feed materials which are foreign to the nitration process.